The present invention relates to a non-crimping polyester monofilament and a process for producing the same. More particularly, the present invention relates to a non-crimping polyester monofilament having an improved surface property and usable as a material monofilament, for ropes, nets, artificial gut, tarpaulins, tents, screens, paragliders and sailcloths, particularly for producing mesh woven fabrics for screen-printing, especially high-mesh high-modulus plain gauzes for screen printing which are required to have a high accuracy, in the production of base plates for printed circuits.
Polyester monofilaments, which may be referred to xe2x80x9cmonofilamentsxe2x80x9d or xe2x80x9cmaterial yarnsxe2x80x9d, are widely employed not only in the clothing field but also in the industrial material field. Particularly, in the industrial material field, the polyester monofilaments are used as material yarns for tire cords, ropes, nets, gut, tarpaulins, tents, screens, paragliders, and sailcloths. The monofilaments for the industrial material use must satisfy severe requirements in adhesive property to rubber, fatigue resistance, dyeability, wear resistance and knot strength.
The requirements in physical properties for the monofilaments has become more and more severe and improvements in rubber-adhesion, fatigue resistance, dyeing property, wear resistance and knot strength of the monofilaments is strongly required.
Particularly, the polyester monofilaments have excellent dimensional stability and thus, in the field of the material yarns for plain gauzes for screen printing, the polyester monofilaments now replace the natural filaments, for example, silk filaments and inorganic filaments, for example, stainless steel filaments.
In the field of the circuit printing for electronic devices, for example, base plates of printed circuits, the degree of integration of the printed circuits is further increased, and thus the accuracy of the circuit printing by the screen printing is strongly required to be increased. Namely, the printing screen for the circuit printing is required to has further increased mesh degree, mechanical strength and modulus. Therefore, the material yarns for the printing screen are required to have further enhanced mechanical strength and modulus and a further decreased thickness. Generally, it is well known that, to increase the mechanical strength and the modulus of the polyester monofilaments, melt-spun, undrawn polyester monofilament is heat-drawn at a high draw ratio to orientate and crystallize the polyester molecules in the monofilament to a great extent.
However, in the procedure for producing the printing screen, the polyester monofilaments are woven into a high density woven fabric to meet with the above-mentioned requirement of a high mesh fabric. In this weaving procedure, the polyester monofilaments are subjected to repeated severe abrasions with a reed of the weaving machine (loom).
Therefore, a portion of the periphery of the monofilament is worn out and fluff or powder-like scum is generated in the weaving machine. Thus the productivity of the printing screen is decreased and the quality of the product is degraded. Also, it is known that the higher the orientation degree and the crystallization degree of the polyester monofilament, the severer the above-mentioned problems in the weaving procedure. When the scums are accumulated in the weaving machine, the weaving procedure is stopped, and when woven printing screen is contaminated with the scum, the contaminating scum causes printing defects to be generated on the printed products in a precision printing procedure.
To prevent or restrict the generation of the scum in the weaving procedure, for example, Japanese Unexamined Patent Publication No. 55-16,948 proposes to employ, as warp yarns, stretch polyester filament yarns having an ultimate elongation of 30 to 60%. However, the high stretch yarns usually have a low modulus and thus cannot meet the requirement of high-strength and high-modulus for the material yarns for the printing screens.
As mentioned above, a high draw ratio is necessary to obtain the high-strength high-modulus polyester monofilament. However, the high draw ratio causes a portion of the polyester resin located in a peripheral part of the resultant drawn polyester monofilament to have a higher degree of orientation of the polyester molecules than that of another portion of the polyester resin located in a center part of the monofilament, and thus the peripheral part of the monofilament is easily partially worn out by abrasion. To solve this problem, there are various proportions of forming the peripheral part of the monofilament by a polymeric melt different from conventional one, to realize both the production of high strength high modulus monofilament and the prevention of scum generation during the weaving prodcedure.
For example, Japanese Unexamined Patent Publication No. 1-132,829 discloses a core-in-sheath type monofilament comprising a core part formed from a polyester and a sheath part formed from a nylon. This core-in-sheath type monofilament has a high mechanical strength and exhibits a high restriction effect on scum generation. However, in this type of monofilament, the sheath part exhibits a high moisture absorption due to the coherent property of nylon and this high moisture absorption of the sheath portion disadvantageously causes the monofilament to exhibit, as a whole, a reduced dimensional stability. Further, since the monofilament is constituted from a polyester sheath part and a nylon core part and polyester and nylon are uncompatible with each other, when the monofilament is repeatedly fatigued under stress during printing procedures the polyester sheath part and the nylon core part may easily separate, at the interface therebetween, from each other.
To solve the above-mentioned problem, Japanese Unexamined Patent Publication No. 2-289,120 provides a core-in-sheath type monofilament having a core part formed from a polyester homopolymer having an intrinsic viscosity of 0.80 and a sheath part formed from a polyethylene glycol-copolymerized polyester having an intrinsic viscosity of 0.67. In the core-in-sheath type composite monofilament, the wearing out of the monofilament due to contact and friction with reed and heald of a weaving machine occurs at the peripheral part of the monofilament. Therefore, the above-mentioned core-in-sheath type monofilament is characterized in that the peripheral part of the monofilament is formed from a polyester copolymer having a low glass transition temperature and exhibiting high resistance to friction and abrasion. Therefore, the strength and modulus of the core-in-sheath type monofilament depend mainly on those of the core part formed from the polyester homopolymer. Thus, in view point of the mechanical properties of the monofilament, it is advantageous that the sheath part formed from the polyester copolymer has a small thickness, in other words, in the cross-section of the monofilament, the ratio of the cross-sectional area of the sheath part to the total cross-sectional area of the monofilament is kept low. However, when the thickness of the sheath part of the monofilament is too low, the core part of the monofilament may be partially exposed to the outside and, simultaneously, since the compatibility of the polyester copolymer for the sheath part with the polyester homopolymer for the core part is low, separation of the sheath part from the core part at the interface therebetween unavoidably occurs. This phenomenon causes the scum-restriction effect of the monofilament and the physical properties and functions of the monofilament to be degraded.
For example, in core-in-sheath type monofilaments for plain gauze for printing screen available in the trade in 1998, the cross-sectional proportion of the sheath part is 30 to 40% which is higher than the range proposed in the above-mentioned Japanese unexamined patent publication.
An object of the present invention is to provide a non-crimping polyester monofilament having a high mechanical strength, a high modulus and a high resistance of a peripheral part thereof to abrasion and capable of having a small thickness, and a process for producing the same with a high efficiency.
Another object of the present invention is to provide a non-crimping polyester monofilament having a high resistance of a peripheral part thereof to abrasion in a weaving procedure, useful for producing plain guaze for precision printing screen having a high mesh, a high mechanical strength and a high modulus, and capable of being formed with a very small thickness, and a process for producing the same with a high efficiency.
The above-mentioned object can be attained by the polyester monofilament of the present invention and the process of the present invention for producing the same.
The non-crimping polyester monofilament of the present invention is formed from a polyester resin, and in the monofilament, the polyester resin has an intrinsic viscosity which varies in distribution in such a manner that the farther the location of a part of the monofilament from the longitudinal axis of the monofilament in the direction at right angles to the longitudinal axis, the lower the intrinsic viscosity of a portion of the polyester resin located in the part of the monofilament, and a portion of the polyester resin located in a peripheral part (p) of the monofilament has an average intrinsic viscosity [xcex7]fxe2x88x92p of 0.6 to 1.1, determined in orthochlorophenol at a temperature of 35xc2x0 C., the peripheral part (p) concentrically surrounding a center part (c) of the monofilament extending along the longitudinal axis of the monofilament.
The process of the present invention for producing a non-crimping polyester monofilament, comprises:
melting a polyester resin having an intrinsic viscosity of 0.8 to 1.3, determined in o-chlorophenol at a temperature of 35xc2x0 C.;
dividing the polyester resin melt into at least two portions;
passing the polyester resin melt portions through at least two passages which cause the intrinsic viscosity of the polyester resin melt portions to be decreased to extents different from each other;
extruding the polyester resin melt portions which are different, from each other, in the intrinsic viscosities thereof, through a melt-spinning orifice, in such a manner that a polyester resin melt portion having a highest intrinsic viscosity is extruded through a center part of the orifice, and a polyester resin melt portion having a lowest intrinsic viscosity is extruded through a peripheral part concentrically surrounding the center part of the orifice, to form a filamentary stream of the polyester resin melt;
drafting and solidifying the resultant filamentary stream of the polyester resin melt to form a monofilament of the polyester resin;
taking up the drafted and solidified polyester monofilament; and
heat-drawing the taken-up undrawn monofilament,
wherein during the extruding step through the heat-drawing step, the polyester resin portions different in intrinsic viscosity from each other are diffused, at the interface portion thereof, into each other, to cause the resultant monofilament to have a distribution of the intrinsic viscosity of the polyester resin such that the farther the location of a part of the monofilament from the longitudinal axis of the monofilament in the direction at right angles to the longitudinal axis, the lower the intrinsic viscosity of a portion of the polyester resin located in the part of the monofilament, and a portion of the polyester resin located in the peripheral part of the monofilament has an average intrinsic viscosity [xcex7]fxe2x88x92p of 0.6 to 1.1 determined in o-chlorophenol at a temperature of 35xc2x0 C.